Optical methods are suitable for sensing flow of fluids in pipes because they are non-invasive and offer high accuracy in a range of speed from millimeters per second up to several kilometers per second. However, traditional flow sensing devices such as laser Doppler velocimeters (“LDVs”) require fluid flowing in a pipe to be seeded with foreign particles in order to maintain a proper signal-to-noise ratio. This feature limits the usefulness of LDVs because many industrial applications such as natural gas or steam pipelines, vent pipes, flare stacks, production of clean industrial gases and medical grade gases, etc. do not allow introduction of any particle contaminants.
Another type of optical velocimeter based on a method known as the “laser-two-focus” (“L2F”) method uses particles naturally present in the flow for measurement of the time interval necessary for a particle to cross two sequential laser beams. Accuracy and turndown ratio (i.e., the ratio of the maximum to minimum flow rates which are measurable within a specified accuracy and repeatability) of the L2F method, however, is affected by the size and concentration of particles in the fluid. This limits practical application of the L2F method because the size and concentration of particles varies over time and at different measurement sites.
Neither LDV nor L2F velocimeters provide an average of the flow velocity across the pipe. This is a common drawback of single point measurement approaches, which could be improved only by adding extra measuring points. Multiple point measurement, however, requires complex optical systems, which increases the overall cost of sensing the flow.
U.S. Pat. No. 6,545,261 (Blake et al.) describes a fiber optic flow meter utilizing a Sagnac interferometer. Beams of light are passed through the flow in opposite directions parallel to the direction that the flow is moving. Due to the Fresnel drag effect, the light moving in the same direction as the flow travels faster than the light moving against the flow. By measuring the phase shift between paired beams traveling along the same path in opposite directions, it is possible to measure fluid velocity in the pipe, provided that the pair or beams are properly aligned. The accuracy increases proportionally with the length of the optical path along the pipe due to accumulation of the phase difference.
However, the longer the distance the light travels inside the pipe, the greater the beam misalignment. The misalignment reduces the signal-to-noise ratio and subsequently increases the measuring error. The beam misalignment is caused mainly by the refraction of the beam in the pipe; the beam oscillates due to turbulence in the flow and heat convection. Blake et al. also describes an alignment system for tracking the beam position at both sides of the pipe and keeping the beam stabilized with help of piezoelectric actuators. This method of velocity measurement requires complex and expensive hardware, including high-speed two-axis piezoelectric positioners. The frequency response of such elements is typically in the range of 10 kHz, which is lower than the frequency range of beam twinkling. Therefore, the alignment system is unable to compensate for rapid oscillation of the beam, and measuring error will be introduced by high frequency components generated by turbulence in the flow. In addition, piezoelectric actuators are subject to high hysteresis and have a limited range of operating temperatures. In particular, the piezoelectric effect is diminished below 0° C. The required temperature stabilization adds complexity and cost to the system because the temperature of the moving fluid can be highly variable (such as in flare stacks).
Cross-correlation methods for non-invasive measurement of fluid flow using optical means are known in the art. Optical methods usually are not adversely affected by high temperature because light sources and photodetectors can be located remotely from the hot measuring zones. U.S. Pat. No. 6,611,319 (Wang) describes an optical flow meter which is based on registration of the light twinkled (scintillated) due to the small changes of the refractive index with changes in temperature. The moving fluid is transilluminated by a single light source and the direct light is measured by two photodetectors spaced apart along the direction of flow. A cross-correlation function between signals from those photodetectors is calculated and a position of its maximum is determined. This position provides the average time which is necessary for the flow to move from one photodetector to the other. Consequently, the ratio of the distance between the photodetectors to the time delay gives an estimate of the average velocity of the flow. This method has low accuracy. The highest accuracy is about 1% according to the device specifications. A need for larger diameter pipe is another drawback of this method. The flow meter can be used for pipes not smaller than one meter in diameter. These factors limit practical application of the twinkling method.
There is the need for a robust, reliable, simple and inexpensive optical flow meter which will provide an accurate measurement of flow rate in pipe of various diameters.